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ix                       CHARACTERS OF  BLEPHANTIDAB                    217
with abundant air cavities in  the  roofing  and other bones.      The
incisors are developed into long tusks., which exist in the upper
jaw alone, in the lower jaw alone, or in both jaws. There are
no canines. The molars are lophodont. The clavicle is absent.
The femur has no third trochanter. The bones of the carpus are
serially arranged and do not interlock. The stomach is simple.
The brain has much convoluted cerebral hemispheres, but the
cerebellum is completely uncovered by them. The intestine is
provided with a wide caecum. The testes are abdominal. The
teats are pectoral in position. The placenta is non-deciduate and
zonary. There are two venae eavae superiores.
The position of the limbs in the Elephant tribe is unique among
living animals : their straightness that is to say, and the absence
or very slight development of angulation at the joints of the
limb bones. This same feature has been observed in the extinct
Dinocerata and in the Titanotheria. It must not, however, be
assumed from the resemblance to these ancient forms that there is
much affinity between them and the Proboscidea, or that the latter
have retained an ancient feature of organisation. The oldest
Ungulates for the most part, and the Creodonts to which they are
undoubtedly related, have much bent limbs. It must be considered/
therefore, that the arrangement obtaining in the Elephants is purely
secondary. Professor Osborn has put forward the reasonable viewI
that fche vertical limbs of all these colossal creatures are due to
" an adaptation designed to transmit the increasing weight *' of
these animals. The huge bulk of the body is better borne by
vertical pillars than by an angulated limb. Other points, however,
such as the. exposure of the cerebellum, the two venae eavae, the
five digits, and the absence of a third trochanter, argue a low
position for the Proboscidea in the Eutherian group.
The group can be readily divided into two families, the
Elephantidae and the Dinotheriidae. We 'will commence with
the former.
The Elephants proper, Elephantidae, differ. from the Dino-
theriidae in, and are characterised by, a number of anatomical
features. They possess long tusks (incisors) either in both jaws,
or, if only in one jaw, in the upper. The molar teeth are very
large—so large that only a few of them are simultaneously in use.
There are but three definable genera of Elephantidae, of wMcli
1 American Nat. February 1900, p. 80.